A growth factor has been purified to apparent homogeneity from human milk and primary human mammary tumors. The factor appears to be a new, as yet underscribed, protein. It has a molecular weight of 62,000 and pI of 4.8. Because of the tissue of origin, we have named it mammary derived growth factor 1, MDGF1. MDGF-1 stimulates the growth of normal mammary cells and mammary tumor cells such as MCF-7. Optimal response is seen at about 10 ng/ml. The factor appears to act synergistically with estrogen since mammary epithelium from estrogen primed animals is responsive while that from unprimed animals is not. 125I-MDGF1 binds specifically to high affinity receptors on cell membranes. In addition to stimulating growth, the factor differentially amplifys collagen synthesis as much as 7 fold. In NRK cells the differential stimulation of collagen synthesis is produced via a MDGF-1 stimulation of collagen mRNA production. Mammary cell responsiveness is substratum dependent, being manifest on stromal collagen or tissue culture plastic but not on a basement membrane collagen substratum. These findings suggest that responsiveness to MDGF-1 might be facilitated as proliferating mammary cells penetrate through the basement membrane and contact the stroma. Such a mechanism might explain how a new basement membrane is synthesized by proliferating epithelium.